Conventionally, the cyclist exerts a force on pedals which are rotatably mounted at the remote ends of cranks, the near or inner ends of the cranks being associated with a horizontal crank spindle which is itself rotatably mounted in the crank case of the bicycle frame.
One or two externally toothed chain wheels (sprockets) are fixed on the rotatable axle in the crank case, one of which engages with the drive chain which is also engaged with a sprocket fixed on the hub of the rear wheel which, as is known, additionally comprises a free-wheeling clutch. Most frequently, two chain wheels of different diameters are provided on the crank spindle, and five sprockets of different diameters are provided on the rear wheel.
The various combinations of the two chain wheels with the five rear sprockets make it possible to obtain ten different speed ratios for utilizing the effort produced by the rider. Selecting of the large chain wheel with a small sprocket gives a high speed ratio, applying a relatively low torque to the rear wheel. Alternatively, selecting the small chain wheel with a large sprocket produces a low speed ratio and hence a larger torque on the rear wheel.